assoc_array: Include rcupdate.h for call_rcu() definition

Include rcupdate.h header to provide call_rcu() definition. This was implicitly
being provided by slab.h file which include srcu.h somewhere in its include
hierarchy which in-turn included rcupdate.h.

Lately, tinification effort added support to remove srcu entirely because of
which we are encountering build errors like

lib/assoc_array.c: In function 'assoc_array_apply_edit':
lib/assoc_array.c:1426:2: error: implicit declaration of function 'call_rcu' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

Fix these by including rcupdate.h explicitly.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reported-by: Scott Wood <scottwood@freescale.com>